Turbocharged diesel engines are common powerplants of trucks that are presently being built. A known turbocharged engine comprises a two-stage turbocharger that comprises high- and low-pressure turbines in series flow relationship in the exhaust system that operate high- and low-pressure compressors in series flow relationship in the intake system to develop boost. The high-pressure turbine of a particular type of two-stage turbocharger has vanes that can be controlled by an actuator to control both torque that operates the high-pressure compressor and exhaust back-pressure. Such a turbocharger is sometimes called a variable geometry turbocharger, or VGT for short.
The high-pressure VGT stage is typically designed to have a relatively smaller size that is optimized for low-end engine performance while the low-pressure stage is typically designed with a relatively larger size for high-end performance. The high-pressure stage has the ability to respond well to transient demands at lower engine speeds and is the main contributor to boost over that speed range. At higher speeds, and at larger loads, the low-pressure stage becomes the main contributor to boost because it can provide the necessary greater air-handling capacity. Over a portion of an engine operating range, the high-pressure stage may however interact with the low-pressure stage in ways that affect turbocharger performance.
Compensation for such interaction can be achieved by the inclusion of two bypass valves, one shunting the high-pressure compressor stage and another shunting the high-pressure turbine stage. By opening in the higher speed and load range to shunt flows around the high-pressure stages, the bypass valves prevent the high-pressure stages from choking the flows.
The operation of each bypass valve is controlled in concert with operation of the other, and their operation is coordinated with control of the VGT vanes. The engine control system processes various data according to algorithms to provide control functions for the VGT vanes and the bypass valves such that exhaust back-pressure and engine boost are regulated in a way deemed appropriate for the manner in which the engine is being operated.
For various reasons that bear on engine performance and/or emission control, the ability to accurately control exhaust back-pressure is important to an engine control strategy. A typical strategy processes various data to develop a data value for a desired set-point for exhaust back-pressure. Changes in engine operation that affect that set-point typically call for the control system to respond promptly and accurately to force the actual exhaust back-pressure to follow the changes in the desired set-point.
In the lowest speed range, exhaust back-pressure can be controlled entirely by control of the VGT vanes. When the engine operating conditions change such that exhaust back-pressure can no longer be controlled solely by the VGT vanes, the bypass valves should open. It is desirable that the transition from VGT control to bypass valve control, and vice versa, occur in ways that avoid interactions between the VGT vanes and the bypass valves that would result in undesired effects on control accuracy, such as delayed response, pressure spikes, etc.